Characters in Arpeggio can equip armor to protect themselves in battle. Suits of armor
are stored in the regular item inventory, instead of getting their own inventory
like weapons; however, just as only one weapon can be equipped at a time, only
one suit of armor can be worn. The suit that is currently worn
does not take up a slot in the inventory, and cannot be stolen
(unless the wearer is currently dead).
Normally, armor takes some of the damage for the wearer, and has its own HP stat, which is not restored by any of the means that characters use to heal their HP. Once the armor's HP reaches 0, it breaks, and the wearer is no longer protected, but may put on a new suit of armor. Thus, armor does not last forever and more instances must be purchased or pilfered, although very rare armor might never break. There might also be some means of repairing armor, such as a smithy in the field.
An armor suit's Defense stat is the amount of damage that the armor takes for the wearer per attack, and thus, the defensive bonus provided to the wearer. Armor works as Defense Power (therefore against physical and magical attacks, but not psychic attacks) unless otherwise stated; some armor defends better against magical attacks than physical attacks, which will be indicated in its description. Armor takes damage for the wearer even when the incoming attack is marked [/] ("Spare Attacks"), thus providing even more protection than inherent Defense Power, but attacks which are marked [X] ("Strike Attacks") deal their full damage to the wearer (ignoring their Defense Power) while also dealing the armor's full Defense value to its own HP. If a non-[X] attack deals less total damage than the amount of defense that a suit of armor provides, then that lesser amount is subtracted from the armor's HP, and no damage is dealt to the wearer; if the wearer's Defense Power stat is larger than the power of the attack being used, then no damage is dealt to the armor or the wearer. If the armor has less HP remaining than the Defense value it normally provides, then it will only provide Defense up to its current HP value. [X] attacks deal the full Defense value to the armor's HP even if the full power of the attack is less than this value.
The Shield Status Benefit has a special effect on armor: because it cuts damage in half rather than subtracting some amount, and because the halving occurs after the armor's Defense value has been subtracted from the damage of the incoming attack, the amount of damage that the armor's own HP takes from the attack is also half of what it would otherwise be while the wearer is Shielded. To make sure you're doing it right, think of the armor as a separate character who is also Shielded, and use the amount of damage that the armor is protecting its wearer from as the Attack Power to be halved.
Also of note is that other Status Benefits like Electrification and Reciprocity, as well as similar conditions such as Spikiness, and damage-dealing Weather Conditions, will often deal 1 damage to a character under certain circumstances: this damage ignores regular Defense Power, but does not ignore armor, meaning that most of the time the armor absorbs this 1 damage for the wearer and the wearer is, until the armor breaks, unhampered by these conditions. This is Arpeggio's explanation for why Koops can use Shell Shot on an Electrified enemy without getting hurt: his shell takes the damage for him. However, Status Problems like Poisoning and Burns still ignore armor completely and deal 1 damage directly to the afflicted character (but none to the armor); conceptually this is because they are "internal" conditions instead of "external" ones.
A suit of armor's Weight stat is how heavy it is, and means two things. One, while the armor is worn, its Weight is added to the wearer's Weight stat (which means the total may exceed 9); and two, a character is incapable of wearing armor with a Weight that exceeds their Strength stat. That being said, wearing armor that has a Weight stat above 0 does not actually decrease the wearer's functional Strength stat, so Strength itself is not hampered. Wearing armor does not increase a character's alcohol tolerance, but does increase the damage taken from a Long Fall.
Naturally, when the players acquire suits of armor by looting defeated enemies, any damage that they themselves (or anyone else for that matter) had inflicted on those suits of armor remains present. Therefore, armor obtained through looting is unlikely to be in pristine condition, though armor that is purchased from stores will generally begin with full HP.